U.S. patent application number 09/853114 was filed with the patent office on 2001-11-01 for in-situ mirror characterization.
Invention is credited to Hill, Henry Allen.
Application Number | 20010035959 09/853114 |
Document ID | / |
Family ID | 26900936 |
Filed Date | 2001-11-01 |
United States Patent
Application |
20010035959 |
Kind Code |
A1 |
Hill, Henry Allen |
November 1, 2001 |
In-situ mirror characterization
Abstract
Interferometric apparatus and methods by which the local surface
characteristics of photolithographic mirrors or the like may be
interferometrically measured in-situ to provide correction signals
for enhanced distance and angular measurement accuracy. Surface
characterizations along one or multiple datum lines in one or more
directions may be made by measuring the angular changes in beams
reflected off the surfaces during scanning operations to determine
local slope and then integrating the slope to arrive at surface
topology. The mirrors may be mounted either on the
photolithographic stages or off the photolithographic stages on a
reference frame.
Inventors: |
Hill, Henry Allen; (Tucson,
AZ) |
Correspondence
Address: |
FRANCIS J. CAUFIELD
6 APOLLO CIRCLE
LEXINGTON
MA
02421-7025
US
|
Family ID: |
26900936 |
Appl. No.: |
09/853114 |
Filed: |
May 10, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60205980 |
May 19, 2000 |
|
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60212405 |
Jun 19, 2000 |
|
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Current U.S.
Class: |
356/500 |
Current CPC
Class: |
G03F 7/70775 20130101;
G03F 7/70591 20130101; G01B 11/2441 20130101; G01B 11/306
20130101 |
Class at
Publication: |
356/500 |
International
Class: |
G01B 011/02 |
Claims
What is claimed is:
1. Interferometric apparatus comprising: means for defining a
reference frame; a translation stage; an electro-mechanical
arrangement for selectively translating said translation stage in
at least one of at least two orthogonal directions with respect to
said reference frame; at least one thin, elongated mirror mounted
in a predetermined manner with respect to said reference frame,
said at least one thin, elongated mirror having a reflecting
surface and a nominal datum line extending along its longitudinal
dimension; at least one interferometer subsystem mounted in a
predetermined manner with respect to said at least one thin,
elongated mirror; adapted to cooperate with said at least one thin,
elongated mirror to measure the displacement of said translation
stage in at least one azimuth; and adapted to measure the local
slope of said at one thin, elongated mirror along and orthogonal to
its datum line and its local displacement normal to said reflecting
surface; control means having a mode of operation for selectively
translating said translation stage, said at least one thin,
elongated mirror and said at least one interferometer subsystem
moving relative to one another in said mode of operation so that
said at least one interferometer subsystem scans said at least one
thin, elongated mirror along its corresponding datum line to
generate a signal containing information indicative of the angular
change and surface departure of said reflecting surface thereof
along with any contributions thereto due to variations present from
said electro-mechanical arrangement per se; and signal and analysis
means for extracting said information contained in said signal and
determining the local shape of said at least one thin, elongated
mirror while said control means is in said mode of operation.
2. The interferometric apparatus of claim 1 wherein said at least
one thin, elongated mirror is mounted to said translation stage for
movement therewith and said at least one interferometer subsystem
is fixedly mounted off said translation stage.
3. The interferometric apparatus of claim 1 wherein said at least
one interferometer subsystem is fixedly mounted to said translation
stage for movement therewith and said at least one thin, elongated
mirror is fixedly mounted off said translation stage.
4. The interferometric apparatus of claim 1 wherein said control
means is structured and arranged to have another mode of operation
in which the motion of said translation stage is measured in at
least one azimuth with respect to said reference frame.
5. The interferometric apparatus of claim 1 comprising at least
two, thin elongated mirrors having reflecting surfaces orthogonally
arranged with respect to one another and each including a nominal
datum line extending along its longitudinal dimension and at least
two interferometer subsystems at least in part mounted off said
translation stage, each of said at least two interferometer
subsystems being adapted to scan a corresponding one of said thin,
elongated mirrors and configured to measure the local slope of the
scanned mirror along and orthogonal to its datum line and its local
displacement normal to said reflecting surface, said control means
being further configured in said mode of operation to selectively
translate said translation stage in one or all of its possible
directions of motion so that at least one of said interferometer
subsystems scans a corresponding one of said thin, elongated
mirrors along its corresponding datum line to generate a signal
containing information indicative of the angular change and surface
departure of its corresponding reflecting surface along with any
contributions thereto due to variations present from said
electro-mechanical arrangement per se while the other of said
interferometer subsystems generates a signal containing at least
information about the angular change of said translation stage,
said signal combining and analysis means extracting information
contained in said signals and determining the local shape of said
at least two thin, elongated mirrors.
6. The apparatus of claim 1 wherein said at least one
interferometer subsystem comprises a single beam, plane mirror
interferometer subsystem.
7. The interferometric apparatus of claim 1 wherein said
interferometric apparatus comprises three orthogonally arranged
thin, elongated mirrors and three corresponding interferometer
subsystems mounted for relative motion with respect to one another
while said control means is in said mode of operation to measure
the local shape of said mirrors in three dimensions.
8. The interferometric apparatus of claim 1 further including a
photolithographic wafer mount located on said translation stage for
movement therewith.
9. The interferometric apparatus of claim 8 further including a
photolithographic exposure unit fixedly mounted to said reference
frame for forming masked patterns on wafers located on said
translation stage.
10. Interferometric method comprising the steps of: defining a
reference frame; providing a translation stage for movement with
respect to said reference frame; selectively translating said
translation stage in at least one of at least two orthogonal
directions with respect to said reference frame; mounting at least
one thin, elongated mirror in a predetermined manner with respect
to said reference frame, said at least one thin, elongated mirror
having a reflecting surface and a nominal datum line extending
along its longitudinal dimension; mounting at least one
interferometer subsystem in a predetermined manner with respect to
said at least one thin, elongated mirror where said at least one
interferometer subsystem is adapted to cooperate with said at least
one thin, elongated mirror to measure the displacement of said
translation stage in at least one azimuth and is also adapted to
measure the local slope of said at one thin, elongated mirror along
and orthogonal to its datum line and its local displacement normal
to said reflecting surface; selectively translating said
translation stage in a mode of operation in which said at least one
thin, elongated mirror and said at least one interferometer
subsystem movie relative to one another in said mode of operation
so that said at least one interferometer subsystem scans said at
least one thin, elongated mirror along its corresponding datum line
to generate a signal containing information indicative of the
angular change and surface departure of said reflecting surface
thereof along with any other contributions thereto due to
variations present during said step of selectively translating said
translation stage; and extracting said information contained in
said signal and determining the local shape of said at least one
thin, elongated mirror while is in said mode of operation.
11. The interferometric method of claim 10 wherein said at least
one thin, elongated mirror is mounted to said translation stage for
movement therewith and said at least one interferometer subsystem
is fixedly mounted off said translation stage.
12. The interferometric method of claim 10 wherein said at least
one interferometer subsystem is fixedly mounted to said translation
stage for movement therewith and said at least one thin, elongated
mirror is fixedly mounted off said translation stage.
13. The interferometric method of claim 10 having another mode of
operation in which the motion of said translation stage is measured
in at least one azimuth with respect to said reference frame.
14. The interferometric method of claim 10 in which there are
provided at least two, thin elongated mirrors having reflecting
surfaces orthogonally arranged with respect to one another with
each including a nominal datum line extending along its
longitudinal dimension and at least two interferometer subsystems
at least in part mounted off said translation stage, each of said
at least two interferometer subsystems being adapted to scan a
corresponding one of said thin, elongated mirrors and configured to
measure the local slope of the scanned mirror along and orthogonal
to its datum line and its local displacement normal to said
reflecting surface, said method being further configured in said
mode of operation to selectively translate said translation stage
in one or all of its possible directions of motion so that at least
one of said interferometer subsystems scans a corresponding one of
said thin, elongated mirrors along its corresponding datum line to
generate a signal containing information indicative of the angular
change and surface departure of its corresponding reflecting
surface along with any contributions thereto due to variations
present from any other contributions present during said step of
selectively translating said translation stage while the other of
said interferometer subsystems generates a signal containing at
least information about the angular change of said translation
stage, said step of extracting information contained in said
signals determining the local shape of said at least two thin,
elongated mirrors.
15. The interferometric method of claim 10 wherein said at least
one interferometer subsystem comprises a single beam, plane mirror
interferometer subsystem.
16. The interferometric method of claim 10 in which there are
provided three orthogonally arranged thin, elongated mirrors and
three corresponding interferometer subsystems mounted for relative
motion with respect to one another while in said mode of operation
to measure the local shape of said mirrors in three dimensions.
17. The interferometric method of claim 10 further including the
step of mounting a photolithographic wafer on said translation
stage for movement therewith.
18. The interferometric method of claim 17 further including
photolithographically exposing said wafer from said reference frame
with masked patterns of illumination.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims priority from U.S. Provisional
Patent Application No. 60/205,980 filed on May 19, 2000 and U.S.
Provisional Patent Application No. 60/212,405 filed on Jun. 19,
2000, the contents of which are incorporated herein by reference in
their entirety.
BACKGROUND OF THE INVENTION
[0002] This invention in general relates to interferometry and in
particular to interferometric apparatus and methods by which the
local surface characteristics of photolithographic stage mirrors or
the like may be interferometrically measured in-situ to provide
correction signals for enhanced distance measurement accuracy.
[0003] Interferometry is a well established metrology used
extensively in microfabrication processes to measure and control a
host of critical dimensions. It is especially important in
manufacturing semiconductors and the like where requirements for
precision are 10 to 40% better than critical dimensions of 0.1
.mu.m or less.
[0004] Integrated circuits made of semiconductor materials are
constructed by successively depositing and patterning layers of
different materials on a silicon wafer while it typically resides
in a flat exposure plane having Cartesian x-y coordinates to which
there is a normal z-direction. The patterning process consists of a
combination of exposure and development of photoresist followed by
etching and doping of the underlying layers followed by the
deposition of subsequent layers. This process results in a complex
and, on the scale of microns, very nonhomogeneous material
structure on the wafer surface.
[0005] Typically each wafer contains multiple copies of the same
pattern called "fields" arrayed on the wafer in a nominally
rectilinear distribution known as the "grid." Often, but not
always, each field corresponds to a single "chip."
[0006] The exposure process consists of projecting the image of the
next layer pattern onto (and into) the photoresist that has been
spun onto the wafer. For an integrated circuit to function properly
each successive projected image must be accurately matched to the
patterns already on the wafer. The process of determining the
position, orientation, and distortion of the patterns already on
the wafer, and then placing them in the correct relation to the
projected image, is termed "alignment." The actual outcome, i.e.,
how accurately each successive patterned layer is matched to the
previous layers, is termed "overlay."
[0007] In general, the alignment process requires both
translational and rotational positioning of the wafer and/or the
projected image as well as some distortion of the image to match
the actual shape of the patterns already present. The fact that the
wafer and the image need to be positioned correctly to get one
pattern on top of the other is obvious. Actual distortion of the
image is often needed as well. Other effects, such as thermal and
vibration, may require compensation as well.
[0008] The net consequence of all this is that the shape of the
first-level pattern printed on the wafer is not ideal and all
subsequent patterns must, to the extent possible, be adjusted to
fit the overall shape of the first-level printed pattern. Different
exposure tools have different capabilities to account for these
effects, but, in general, the distortions or shape variations that
can be accounted for include x and y magnification and skew. These
distortions, when combined with translation and rotation, make up
the complete set of linear transformations in the plane.
[0009] Since the problem is to successively match the projected
image to the patterns already on the wafer, and not simply to
position the wafer itself, the exposure tool must effectively be
able to detect or infer the relative position, orientation, and
distortion of both the wafer patterns themselves and the projected
image.
[0010] It is difficult to directly sense circuit patterns
themselves, and therefore, alignment is accomplished by adding
fiducial marks or "alignment marks" to the circuit patterns. These
alignment marks can be used to determine the reticle position,
orientation, and distortion and/or the projected image position,
orientation, and distortion. They can also be printed on the wafer
along with the circuit pattern and hence can be used to determine
the wafer pattern position, orientation, and distortion. Alignment
marks generally consist of one or more clear or opaque lines on the
reticle, which then become "trenches" or "mesas" when printed on
the wafer. But more complex structures such as gratings, which are
simply periodic arrays of trenches and/or mesas, and checkerboard
patterns are also used. Alignment marks are usually located either
along the edges of "kerf" of each field or a few "master marks" are
distributed across the wafer. Although alignment marks are
necessary, they are not part of the chip circuitry and therefore,
from the chip maker's point of view, they waste valuable wafer area
or "real estate." This drives alignment marks to be as small as
possible, and they are often less than a few hundred micrometers on
a side.
[0011] Alignment sensors are incorporated into the exposure tool to
"see" alignment marks. Generally there are separate sensors for the
wafer, the reticle, and/or the projected image itself. Depending on
the overall alignment strategy, these sensors may be entirety
separate systems or they may be effectively combined into a single
sensor. For example, a sensor that can see the projected image
directly would nominally be "blind" with respect to wafer marks and
hence a separate wafer sensor is required. But a sensor that
"looks" at the wafer through the reticle alignment marks themselves
is essentially performing reticle and wafer alignment
simultaneously and hence no separate reticle sensor is necessary.
Note that in this case the positions of the alignment marks in the
projected image are being inferred from the positions of the
reticle alignment marks and a careful calibration of reticle to
image positions must have been performed before the alignment
step.
[0012] Furthermore, as implied above, essentially all exposure
tools use sensors that detect the wafer alignment marks optically.
That is, the sensors project light at one or more wavelengths onto
the wafer and detect the scattering/diffraction from the alignment
marks as a function of position in the wafer plane. Many types of
alignment sensors are in common use and their optical
configurations cover the full spectrum from simple microscopes to
heterodyne grating interferometers. Also, since different sensor
configurations operate better or worse on given wafer types, most
exposure tools carry more than one sensor configuration to allow
for good overlay on the widest possible range of wafer types.
[0013] The overall job of an alignment sensor is to determine the
position of each of a given subset of all the alignment marks on a
wafer in a coordinate system fixed with respect to the exposure
tool. These position data are then used in either of two generic
ways termed "global" and "field-by-field" to perform alignment. In
global alignment the marks in only a few fields are located by the
alignment sensor(s) and the data are combined in a best-fit sense
to determine the optimum alignment of all the fields on the wafer.
In field-by-field alignment the data collected from a single field
are used to align only that field. Global alignment is usually both
faster, because not all the fields on the wafer are located, and
less sensitive to noise, because it combines all the data together
to find a best overall fit. But, since the results of the best fit
are used in a feed-forward or dead reckoning approach, it does rely
on the overall optomechanical stability of the exposure tool.
[0014] Alignment is generally implemented as a two-step process;
that is, a fine alignment step with an accuracy of tens of
nanometers follows an initial coarse alignment step with an
accuracy of micrometers, and alignment requires positioning the
wafer in all six degrees of freedom: three translation and three
rotation. But adjusting the wafer so that it lies in the projected
image plane, i.e., leveling and focusing the wafer, which involves
one translational degree of freedom (motion along the optic axis,
the z-axis or a parallel normal to the x-y wafer orientation) and
two rotational degrees of freedom (orienting the plane of the wafer
to be parallel to the projected image plane), is generally
considered separate from alignment. Only in-plane translation (two
degrees of freedom) and rotation about the projection optic axis
(one degree of freedom) are commonly meant when referring to
alignment. The reason for this separation in nomenclature is the
difference in accuracy required. The accuracy required for in-plane
translation and rotation generally needs to be on the order of
several tens of nanometers or about 20 to 30% of the minimum
feature size or critical dimension (CD) to be printed on the wafer.
Current state-of-the-art CD values are on the order of several
hundred nanometers, and thus, the required alignment accuracy is
less than 100 nm. On the other hand, the accuracy required for
out-of-plane translation and rotation is related to the total
usable depth of focus of the exposure tool, which is generally
close to the CD value. Thus, out-of-plane focusing and leveling the
wafer require less accuracy than in-plane alignment. Also, the
sensors for focusing and leveling are usually completely separate
from the "alignment sensors" and focusing and leveling do not
usually rely on patterns on the wafer. Only the wafer surface or
its surrogate needs to be sensed. Nevertheless, this is still a
substantial task requiring, among other things, precise knowledge
about the vertical position (the altitude) of the optical
projection system above the wafer.
[0015] To achieve alignment, it is known to use dynamic
interferometers in which distance measurements are enhanced through
the use of dynamic elements whose angular orientation is controlled
via feedback arrangements to assure that beams carrying distance
information are properly aligned to provide optimal signal. Such
interferometers are shown, for example, in International
Application No. PCT/US00/12097 filed May 5, 2000, and entitled
"Interferometry Systems Having a Dynamic Beam-Steering Assembly For
Measuring Angle and Distance" by Henry A. Hill. However, even with
dynamic interferometers the shape of various reflecting elements
impacts on the achievable accuracy in distance measurements and
impacts on the achievable accuracy in angle measurements, because
for the latter local slope changes influence beam directions, as
stage mirrors undergo their various motions. Typically, the shape
of such reflecting elements, such as thin high aspect ratio
mirrors, is characterized off-stage and, if judged to be of
adequate consistency, are then mounted on-stage. However, this is
often unacceptable because the mounting process itself distorts the
shape of the element compared with its inspected shape, and this
change in shape can introduce measurement errors.
[0016] Accordingly, it is a major object of the present invention
to provide interferometric apparatus and methods by which the
shapes of on-stage reflecting elements, such as thin high aspect
ratio mirrors, may be measured in-situ, after mounting, to develop
correction signals that compensate for errors in optical path
lengths and in beam directions related to shapes of reflecting
surfaces.
[0017] It is another object of the present invention to provide
interferometric apparatus and methods by which the shapes of
on-stage reflecting elements, such as thin high aspect ratio
mirrors, may be measured in-situ, after mounting, to develop
correction signals that compensate for errors in optical path
lengths and in beam directions related to shapes of reflecting
surfaces arranged in orthogonal planes.
[0018] It is yet another object of the present invention to exploit
information generated from the operating properties of dynamic
interferometers by which the shapes of on-stage reflecting
elements, such as thin high aspect ratio mirrors, may be measured
in-situ, after mounting, to develop correction signals that
compensate for errors in optical path lengths and in beam
directions related to shapes of reflecting surfaces.
[0019] It is yet another object of the present invention to provide
interferometric apparatus and methods by which the shapes of
off-stage reflecting elements, such as thin high aspect ratio
mirrors, may be measured in-situ, after mounting, to develop
correction signals that compensate for errors in optical path
lengths and in beam directions related to shapes of reflecting
surfaces.
[0020] Other objects of the present invention will, in part, be
obvious and will, in part, appear hereinafter when reading the
following detailed description in conjunction with the
drawings.
SUMMARY OF THE INVENTION
[0021] Interferometric apparatus and methods by which the local
surface characteristics of photolithographic mirrors or the like
may be interferometrically measured in-situ to provide correction
signals for enhanced distance and angular measurement accuracy.
Surface characterizations along one or multiple datum lines in one
or more directions may be made by measuring the angular changes in
beams reflected off the surfaces during scanning operations to
determine local slope and then integrating the slope to arrive at
surface topology. The mirrors may be mounted either on
photolithographic stages or off the photolithographic stages on a
reference frame. For the simplest case one dynamic beam-steering
assembly or interferometer subsystem is employed for this purpose.
For mirror characterization in two orthogonal directions, at least
two dynamic beam-stearing assemblies are used. One produces a
signal that contains information about the change in slope of the
mirror surface along the datum line and orthogonal to it and the
other produces a signal that contains information about the angular
orientation of the stage on which the mirror is mounted. These two
signals are combined to extract information about the slope of the
mirror along its datum line and orthogonal to it. The slope is then
integrated to obtain topography as a function of displacement.
Single beam interferometers are preferred because they can measure
pitch, yaw, and displacement with only a single beam to the mirror.
Measurements can be made of a plurality of mirrors facing in
mutually orthogonal directions by sequentially holding one or more
fixed relative to their elongated surfaces while translating the
third along its elongated dimension and repeating the process.
Alternatively, all mirrors can be moved together to obtain relative
mirror topography. Three beam-stearing assemblies may be used to
fully characterize three corresponding mutually orthogonal mirrors
and beam-stearing or interferometer subsystems may be mounted on or
off the translation stage.
[0022] Once the mirror's in-situ topography is established, it is
stored in look-up-tables (LUTs), or the like, to provide real-time
error correction signals to improve precision during normal
operation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] The structure, operation, and methodology of the invention,
together with other objects and advantages thereof, may best be
understood by reading the detailed description in connection with
the drawings in which each part has an assigned numeral that
identifies it wherever it appears in the various drawings and
wherein:
[0024] FIG. 1 is a diagrammatic perspective view of an
interferometric apparatus employing a pair of orthogonally arranged
dynamic interferometers by which the shape of an on-stage mounted
elongated object mirror may be characterized in situ along a datum
line as the stage is translated in one direction;
[0025] FIG. 2 is a diagrammatic perspective view of an
interferometric apparatus employing a pair of orthogonally arranged
dynamic interferometers by which the shapes of on-stage
orthogonally mounted elongated object mirrors may be characterized
in situ along datum lines associated with each mirror as the stage
is translated preferably first in one direction and then in an
orthogonal direction or by which the relative shapes of the mirrors
may be obtained by simultaneous motion of the stage along
orthogonal directions;
[0026] FIG. 3 is a diagrammatic perspective view of an
interferometric apparatus employing three orthogonally arranged
dynamic interferometers by which the shapes of on-stage
orthogonally mounted elongated object mirrors may be characterized
in situ along multiple orthogonal datum lines associated with each
mirror as the stage is translated along three orthogonal
directions;
[0027] FIGS. 4a and 4b are, respectively, diagrammatic top and an
elevational views of an interferometer for use in the apparatus of
FIG. 3;
[0028] FIG. 5 is a flow chart in accordance with a method of the
invention; and
[0029] FIG. 6 is a diagrammatic perspective view of an
interferometric apparatus employing three on-stage mounted
orthogonally arranged dynamic interferometers by which the shapes
of corresponding off-stage orthogonally mounted elongated object
mirrors may be characterized in situ along multiple orthogonal
datum lines associated with each mirror as the stage is translated
along three orthogonal directions.
DETAILED DESCRIPTION
[0030] Reference is now made to FIG. 1 which is a diagrammatic
perspective view of an interferometric system 15 that employs a
pair of orthogonally arranged dynamic interferometers or
interferometer subsystems by which the shape of an on-stage mounted
elongated object mirror may be characterized in situ along a datum
line. As shown in FIG. 1, system 15 comprises a stage 16 that
preferably forms part of a photolithographic apparatus for
fabricating semiconductor products such as integrated circuits or
chips. Affixed to stage 16 is a thin, high aspect ratio planar
mirror 50 having a y-z reflective surface 51 elongated in the
y-direction. Also, fixedly mounted to stage 16 is another thin,
high aspect ratio planar mirror 60 having an x-z reflective surface
61 elongated in the x-direction. Mirrors 50 and 60 are mounted on
stage 16 so that their reflective surfaces, 51 and 61,
respectively, are nominally orthogonal to one another. Stage 16 is
otherwise mounted in a well-known manner for nominally plane
translation but may experience small angular rotations about the x,
y, and z axes due to bearing and drive mechanism tolerances. In
normal operation, system 15 is adapted to be operated for
displacement in only the y-direction.
[0031] Fixedly mounted off-stage is a single beam dynamic
interferometer (or interferometer subsystem) 10 for measuring
angular rotation of stage 16, and thus planar mirror reflecting
surface 51, about the y and z axes as stage 16 translates in the
y-direction. To accomplish this, dynamic interferometer 10 is
structured and arranged in the manner described in aforementioned
PCT Patent Application filed May 5, 2000 and entitled
"Interferometry Systems Having a Dynamic Beam-Steering Assembly For
Measuring Angle and Distance" by Henry A. Hill which is
incorporated herein by reference in its entirety. As described in
that application, mirrors are provided with beam steering
capability by which bothersome stage rotations are measured to
provide feedback signals that are used to maintain beams on paths
that are normal to the mirrors. Here, the return beam component of
beam 12 is monitored, and its angle is measured via interferometric
apparatus such as that described in U.S. Patent Application No.
60/201,457 filed on May 3, 2000 in the name of Henry Allen Hill and
entitled "Apparatus And Method(s) For Measuring And/Or Controlling
Differential Paths Of Light Beams", the entirely of which is
incorporated herein by reference.
[0032] Input beam 12 preferably comprises two orthogonally
polarized components having a difference in frequencies
.function..sub.1. A source of input beam 12 such as a laser can be
any of a variety of frequency modulation apparatus and/or lasers.
For example, the laser can be a gas laser, e.g., a HeNe laser,
stabilized in any of a variety of conventional techniques known to
those skilled in the art, see for example, T. Baer et al.,
"Frequency Stabilization of a 0.633 .mu.m He--Ne-longitudinal
Zeeman Laser," Applied Optics, 19, 3173-3177 (1980); Burgwald et
al., U.S. Pat. No. 3,889,207, issued Jun. 10, 1975; and Sandstrom
et al., U.S. Pat. No. 3,662,279, issued May 9, 1972. Alternatively,
the laser can be a diode laser frequency stabilized in one of a
variety of conventional techniques known to those skilled in the
art, see for example, T. Okoshi and K. Kikuchi, "Frequency
Stabilization of Semiconductor Lasers for Heterodyne-type Optical
Communication Systems," Electronic Letters, 16, 179-181 (1980) and
S. Yamaqguchi and M. Suzuki, "Simultaneous Stabilization of the
Frequency and Power of an AlGaAs Semiconductor Laser by Use of the
Optogalvanic Effect of Krypton," IEEE J. Quantum Electronics,
QE-19, 1514-1519 (1983).
[0033] Two optical frequencies may be produced by one of the
following techniques: (1) use of a Zeeman split laser, see for
example, Bagley et al., U.S. Pat. No. 3,458,259, issued Jul. 29,
1969; G. Bouwhuis, "Interferometrie Mit Gaslasers," Ned. T.
Natuurk, 34, 225-232 (August 1968); Bagley et al., U.S. Pat. No.
3,656,853, issued Apr. 18, 1972; and H. Matsumoto, "Recent
interferometric measurements using stabilized lasers," Precision
Engineering, 6(2), 87-94 (1984); (2) use of a pair of
acousto-optical Bragg cells, see for example, Y. Ohtsuka and K.
Itoh, "Two-frequency Laser Interferometer for Small Displacement
Measurements in a Low Frequency Range," Applied Optics, 18(2),
219-224 (1979); N. Massie et al., "Measuring Laser Flow Fields With
a 64-Channel Heterodyne Interferometer," Applied Optics, 22(14),
2141-2151 (1983); Y. Ohtsuka and M. Tsubokawa, "Dynamic
Two-frequency Interferometry for Small Displacement Measurements,"
Optics and Laser Technology, 16, 25-29 (1984); H. Matsumoto, ibid.;
P. Dirksen, et al., U.S. Pat. No. 5,485,272, issued Jan. 16, 1996;
N. A. Riza and M. M. K. Howlader, "Acousto-optic system for the
generation and control of tunable low-frequency signals,"0 Opt.
Eng., 35(4), 920-925 (1996); (3) use of a single acousto-optic
Bragg cell, see for example, G. E. Sommargren, commonly owned U.S.
Pat. No. 4,684,828, issued Aug. 4, 1987; G. E. Sommargren, commonly
owned U.S. Pat. No. 4,687,958, issued Aug. 18, 1987; P. Dirksen, et
al., ibid.; (4) use of two longitudinal modes of a randomly
polarized HeNe laser, see for example, J. B. Ferguson and R. H.
Morris, "Single Mode Collapse in 6328 .ANG. HeNe Lasers," Applied
Optics, 17(18), 2924-2929 (1978); (5) use of birefringent elements
or the like internal to the laser, see for example, V. Evtuhov and
A. E. Siegman, "A "Twisted-Mode" Technique for Obtaining Axially
Uniform Energy Density in a Laser Cavity," Applied Optics, 4(1),
142-143 (1965); or the use of the systems described in U.S. patent
application with Ser. No. 09/061,928 filed Apr. 17, 1998 entitled
"Apparatus to Transform Two Non-Parallel Propagating Optical Beam
Components into Two Orthogonally Polarized Beam Components" by H.
A. Hill, the contents of which are incorporated herein by
reference.
[0034] The specific device used for the source of beam 12 will
determine the diameter and divergence of beam 12. For some sources,
e.g., a diode laser, it will likely be necessary to use
conventional beam shaping optics, e.g., a conventional microscope
objective, to provide beam 12 with a suitable diameter and
divergence for elements that follow. When the source is a HeNe
laser, for example, beam-shaping optics may not be required.
[0035] Another dynamic interferometer 20, preferably of the same
design as that of interferometer 10, is fixedly mounted off-stage
to measure the angular rotation of stage 16 about the x and z axes.
To achieve this, interferometer 20 projects a beam 22 on to mirror
surface 61. A return component of beam 22 is sent to an angle
measuring interferometer as described above. Beam 22 is similarly
generated as was beam 12.
[0036] While system 15 is normally operated to measure y
translation, it is operated in a special mirror characterization
mode to measure the shape of mirror surface 51 in situ along a
datum line thereof. In the mirror characterization mode, stage 16
is translated in the y-direction so that the input beam 12 scans
the mirror surface 51 along a datum line and generates a signal
containing information indicative of its angular orientation and
surface departure in the x-direction and z-direction, along with
any contributions due to variations in the translation mechanism
for moving stage 16. Simultaneous with translation of stage 16 in
the y-direction, interferometer 20 monitors a single point on
mirror 61 corresponding to the intercept point of beam 22 with
reflecting surface 61. This step permits measurement of the
rotation of stage 16 due to mechanical contributions of its
translation mechanism, such as bearings, drive mechanisms, and the
like. With this information, two signals are generated. The first
from interferometer 10 which contains information about the change
in slope of the mirror surface 51 along a datum line and orthogonal
to the datum line, and the second from interferometer 20 which
contains information about the angular orientation of stage 16.
These two signals are combined to extract information only about
the slope of mirror 51 along its datum line and orthogonal to its
datum line, i.e., dx/dy and dx/dz. dx/dy is then integrated to
obtain the x as a function of y. Thus, by measuring the direction
of the change of the output beam 12 in the x-y and x-z planes and
accounting for contributions to those changes brought about by
changes in stage rotations, the shape of mirror surface 51 can be
determined along a datum line and the slope dx/dz can be determined
along the datum line while it is mounted in its working
environment.
[0037] Single beam interferometers are preferred for this
application because they can measure pitch, yaw, and distance (P,
Y, and D) with only a single beam going to the stage mirror 50.
Without changing the normal operation, one can extract in-situ
information about mirror shape with no additional hardware
changes.
[0038] However, the second measurement in a second direction is
required because with translation in the y-direction, stage
bearings and the like cause the stage to wobble introducing large
errors in orientation. Therefore, use is made of mirror surface 61
to measure the deviation or change in orientation of the stage by
looking at the return beam part of 22, also done with a preferably
dynamic interferometer.
[0039] An important feature of the use of single beam
interferometers for this application is it contains all spatial
frequencies up to the cutoff frequency given by 1/d, where d is the
beam diameter whereas use of a double beam interferometer, such as
the HSPMI, would cause loss of all spatial frequencies that have
wavelengths equal to the beam spacing of the two double beams or
harmonics thereof so the shape could not be recovered.
[0040] It will evident to those skilled in the art that the second
interferometer 20 could be another form of angle measuring
interferometer including multiple beam interferometers (not shown)
but of the type shown and described in, for example, "Differential
Interferometer Arrangements for Distance and Angle Measurements:
Principles, Advantages, and Applications, C. Zanoni, VDI Berichte
NR. 749, (1989), the contents of which are included herein by
reference in its entirety, without departing from the scope or
spirit of the present invention.
[0041] Reference is now made to FIG. 2 which is a diagrammatic
perspective view of an interferometric apparatus depicted as system
115. System 115 employs a pair of orthogonally arranged dynamic
interferometers by which the shapes of on-stage orthogonally
mounted elongated object mirrors may be characterized in situ along
datum lines associated with each mirror as a stage is translated
first in one direction and then in an orthogonal direction or by
which the relative shapes of the mirrors may be obtained by
simultaneous motion of the stage along orthogonal directions.
[0042] As seen in FIG. 2, system 115 comprises a stage, again 16,
mounted for plane translation but now normally operated to measure
both x and y motion. A thin, high aspect ratio mirror 150 having a
mirror surface 151 elongated in the y-direction is affixed to stage
16, and a thin, high aspect ratio mirror 160 having an elongated
reflecting surface 161, elongated in the x-direction, is also
fixedly mounted to stage 16 and nominally orthogonal to mirror
150.
[0043] System 115 may also be operated in one of two mirror
characterization modes to measure the surfaces 151 and 161 in situ.
In a first mirror characterization mode, system 115 is operated in
the manner of the mirror characterization mode of system 15 in FIG.
1 to obtain the shape of surface 151. Then, stage 16 is moved in
the x-direction, holding the y-translation fixed to obtain the
shape of mirror 161 in a manner analogous to that for obtaining the
shape of mirror 151. Thus, this is a two step operation.
[0044] In a second mirror characterization mode, stage 16 can be
moved in x and y simultaneously. However, only the relationships
between the shapes of mirror surfaces may be obtained. Only limited
information would be obtained using this mode, but if this
information is sufficiently for the intended downstream use, this
mode eliminates one step in the previous process.
[0045] In connection with normal operation of both the embodiments
of FIGS. 1 and 2 the goal is to obtain information about the shape
of the mirrors so that this information can be used to correct for
the influence of the mirror shapes on the precision with which
distance can be measured. In this regard, a distance correction
algorithm may be used which can be implemented with a look up table
(LUT) or polynomial or Fourier series closed form approximation to
adjust distance measurements. Corrections of the order of {fraction
(1/10)} of a nanometer are possible.
[0046] FIG. 3 is a diagrammatic perspective view of an
interferometric apparatus employing three orthogonally arranged
dynamic interferometers by which the shapes of on-stage
orthogonally mounted elongated object mirrors may be characterized
in situ along multiple orthogonal datum lines (the x-z, x-y, and
y-z planes) associated with each mirror as a stage is translated
along three orthogonal directions, x, y, and z.
[0047] Referring now to FIG. 3, the apparatus of this embodiment is
shown as a system 202 that comprises stage 16 atop of which is
fixedly mounted a plane mirror 270 and a plane mirror 260. Plane
mirror 260 has a reflecting surface 261 oriented in the x-z plane
and elongated in the x-direction. Plane mirror 270 has a reflecting
surface 271 oriented in the y-z plane and elongated in the
y-direction. Mirror 270 also has a top reflecting surface 272
oriented in the x-y plane and elongated in the y-direction.
[0048] Fixedly mounted in a reference body (not shown) is an
elongated plane mirror 280 having a lower reflective surface facing
downwardly, towards stage 16. Fixedly mounted with respect to a
portion of stage 16 making translations in only the x direction is
a single beam interferometer 231 that is adapted to measure the
vertical separation or altitude between mirror surface 272 and the
underside of mirror 280.
[0049] Single beam interferometer 210 having output and return beam
components in beam 212 measures x and pitch and yaw about the y and
z axes as before. Single beam interferometer 220 having output and
return beam components in beam 222 measures y and pitch and yaw
about the x and z axes, respectively, also as before.
[0050] At any altitude the x and y profiles of mirrors 272 and 260
may be measured using the procedures previously described. In
addition to this, however, this embodiment permits the x and y
shapes of mirrors 260 and 270 to also be measured at different
altitudes. For example, the x and y shapes may be determined at one
altitude of stage 16 and then at another that may be vertically
displaced, say 4 to 5 mm, above or below the first. To do this,
angular changes in stage 16 introduced by motion in the z-direction
must be taken into account for optimal precision.
[0051] Interferometer 231 is adapted in a manner to be described to
be sensitive to changes in orientation of the stage 16 by virtue of
it being a single beam interferometer, which makes a single pass of
surfaces 280 and 272, and is otherwise configured to measure pitch
and yaw for beam 233. Source/detector 230 feeds interferometer 231
(See FIGS. 4a and 4b). Therefore, if stage 16 rotates about the x
or y axis, during a translation in the z direction interferometer
231 corrects for that. If beam 233 rolls about the x-axis for beam
212 and rolls about the y axis for beam 222 correction is also
present. With that information for movement in the z direction,
rotation of the stage 16 can be determined with motion in z
compensated for such that surfaces 271 and 261 can be mapped in the
in z-direction as well as y and x directions.
[0052] It will also be evident to those skilled in the art that the
shape of surface 272 can also be obtained in the process of
determining the shapes of surfaces 261 and 271.
[0053] FIGS. 4a and 4b are, respectively, diagrammatic top and an
elevational views of an interferometer 231 for use in the system
202 of FIG. 3. As seen there, interferometer 231 comprises a first
polarizing beam splitter 300 (PBS) having a polarizing beam
splitter layer 302 arranged perpendicular to the paper. PBS 300 is
followed by a PBS 312 having PBS layer 324 oriented at right angles
to PBS layer 302. PBS 312 is followed by a quarter-wave plate 314
and then a Porro prism 316.
[0054] PBS 300 has on one side a quarter-wave plate 304 atop of
which sits a mirror reflecting surface 306, and on the opposite
side of PBS 300 is provided with a quarter-wave plate 308 on which
sits a reflective surface 310.
[0055] PBS 312 has a quarter-wave plate 326 on its top surface and
another quarter-wave plate 330 on the bottom side. Mirrors 280 and
270 reside above and below quarter-wave plates 326 and 330,
respectively.
[0056] A third PBS 318 is provided at the output end of PBS 300 and
includes a PBS layer 319. The return component of beam 232 is split
by PBS 318 into two beams 343 and 345 that are sent to
photodetectors 322 and 320, respectively, to be converted to
electrical signals for further analysis.
[0057] With this arrangement, if interferometer 231 rotates it
doesn't change the orientation of the output beam. However, if
either mirror 270 or 280 rotate, the corresponding angles will be
measured.
[0058] The top view of interferometer in FIG. 4a depicts the path
which the reference beam experiences as it travels through the
interferometer 231 and the elevational view of FIG. 4b depicts the
path which the measurement beam experiences as it travels through
interferometer 231.
[0059] Having described apparatus by which a stage mirror may be
characterized in situ, attention is now directed to FIG. 5 which
shows a flowchart for a method for characterizing stage mirror
topography in situ. As seen there, the method is first started in
block 400, preferably with the stage 16 in a park position. The
next step is to mount an elongated plane object mirror on a
translation stage for plane motion as shown in block 402. This is
followed by the step of directing a single beam from an
interferometer at the object mirror. Next, the stage is moved along
the elongated direction of the mirror while the beam is directed at
it so that the beam scans the mirror along a datum line as shown in
block 406. Following this, the return beam from the mirror is
monitored and the change in angle of the return beam is measured
while the mirror is scanned to generate a signal containing
information about the local slope of the mirror surface along the
datum line as shown in block 408. Then, the stage angular
orientation is measured by directing a single beam from another
orthogonally positioned interferometer at a point on the stage that
does not translate as the stage moves in the direction of the long
dimension of the mirror as shown in block 410. Following this, the
signal from the first interferometer is combined with the measured
stage orientation information to determine the local slope of the
mirror surface as a function of stage displacement. Then in block
414, the slope information is integrated to obtain the mirror
topography along the datum line. Finally, the process may be
repeated as shown in block 416 to map either another orthogonally
positioned stage mirror or to perform scanning along datum lines on
the same mirror displaced from initial datum lines in the
z-direction.
[0060] It will be appreciated that the foregoing process may be
implemented via a suitably programmed general purpose computer or
via dedicated microprocessors that additionally may be used to
exercise overall control of system hardware elements, provide a
user interface for system control and human intervention, and
perform general housekeeping functions.
[0061] Having described the various embodiments, it will be obvious
to those skilled in the relevant art how to make additional changes
based on the teachings of the invention and all such changes are
intended to be within the scope of the invention. For example, It
is known in the metrology of lithography tool wafer stages to also
place an interferometer on the wafer stage and place an associated
bar mirror off the wafer stage on a reference frame of the
lithography tool. See, for example, commonly owned U.S. Pat. No.
5,724,136 entitled "Interferometric Apparatus For Measuring Motions
Of A Stage Relative to Fixed Reflectors" issued March 1998 by Carl
A. Zanoni and U.S. Pat. No. 5,757,160 entitled "Moving
Interferometer Wafer Stage" issued May 1998 by Justin Kreuzer, the
contents of both patent applications incorporated herein by
reference.
[0062] The methods and apparatus described hereinabove may also be
used to characterize in situ the figure of a bar mirror located off
a wafer stage with a dynamic interferometer used as the
interferometer located on the wafer stage. Accordingly, for each of
the foregoing embodiments relating to characterizing the figure(s)
of bar mirror(s) with measuring surface(s) orientated orthogonal to
the plane of a wafer on the wafer stage, there corresponds a set of
embodiments with the bar mirror(s) located off the wafer stage
fixed to a reference frame of a lithography tool and one or more
dynamic interferometers located on the wafer stage. An example of
such an embodiment may be seen in FIG. 6 to which reference is now
made.
[0063] FIG. 6 is a diagrammatic perspective view of an
interferometric apparatus 602 employing three on-stage,
orthogonally arranged dynamic interferometers or interferometer
subsystems by which the shapes of off-stage orthogonally mounted,
thin, elongated object mirrors and an on-stage mounted, thin,
elongated mirror may be characterized in situ along multiple datum
lines (preferably in the x-z, x-y, and y-z planes) associated with
each mirror as a translation stage 616 is translated along three
orthogonal directions, x, y, and z. As will be appreciated, each
interferometer subsystem in combination with an associated mirror
or mirrors is an interferometer used principally for measuring the
displacement of the translation stage 616 so that a wafer 604 held
in position on stage 616 by a wafer holder 603 can be precisely
positioned in an exposing beam 606 generated by a well-known
exposure unit 601 that is mounted with a reference frame 600
(partially shown). The interferometer subsystems are preferably
single beam, plane mirror interferometers, although this is not
essential to the operation of the invention.
[0064] Referring now to FIG. 6, it can be seen that system 602
comprises translation stage 616 atop of which is fixedly mounted an
interferometer subsystem 610 and an interferometer subsystem 620.
Plane mirrors 650 and 670 are fixedly mounted to reference frame
600. Plane mirror 650 has a reflecting surface 661 oriented
substantially in the x-z plane and elongated substantially in the
x-direction. Plane mirror 670 has a reflecting surface 671 oriented
substantially in the y-z plane and elongated substantially in the
y-direction.
[0065] A mirror 680 is fixedly attached to the top of translation
stage 616 and also has a top reflecting surface 682 oriented
substantially in the x-y plane and elongated substantially in the
y-direction.
[0066] Fixedly mounted in reference frame 600 (again partially
shown) is an elongated plane mirror 690 having a lower reflective
surface facing downwardly, towards stage 616. Fixedly mounted with
respect to a portion of stage 616 making translations substantially
in only the x direction is a single beam interferometer 631 that is
adapted to measure the vertical separation or altitude between
mirror surface 682 and the underside of mirror 690.
[0067] Single beam interferometer 610, having output and return
beam components in beam 612, measures displacement substantially in
the x direction and pitch and yaw substantially about the y and z
axes, respectively, as before. Single beam interferometer 620,
having output and return beam components in beam 622, measures
displacement substantially in the y and pitch and yaw substantially
about the x and z axes, respectively, also as before.
[0068] At any altitude the x and y profiles of mirrors 650 and 670
may be measured using the procedures previously described. In
addition to this, however, this embodiment permits the x and y
shapes of mirrors 650 and 670 to also be measured at different
altitudes and as a function of altitude. For example, the x and y
shapes may be determined at one altitude of stage 616 and then at
another that may be vertically displaced, say 4 to 5 mm, above or
below the first. To do this, angular changes in stage 616
introduced by motion in the z-direction must be taken into account
for optimal precision.
[0069] Interferometer subsystem 631 is adapted in a manner to be
described to be sensitive to changes in orientation of the stage
616 by virtue of it being a single beam interferometer, which makes
a single pass to the undersurface of 690 and to surface 682, and is
otherwise configured to measure pitch and yaw for beam 633 relative
to pitch and yaw for beam 634. Therefore, if stage 616 rotates
about the x or y axis, during a translation in the z direction
interferometer 631 corrects for that. With that information for
movement in the z direction, rotation of the stage 616 can be
determined with motion in z compensated for such that surfaces 671
and 661 can be mapped in the in z-direction as well as y and x
directions. A source/detector 630 feeds interferometer subsystem
631 in the manner described in connection with the apparatus of
FIGS. 4a and 4b which is analogous.
[0070] It will also be evident to those skilled in the art that the
shape of surface 682 and the underside of mirror 690 can also be
obtained in the process of determining the shapes of surfaces 661
and 671.
[0071] From the foregoing, it will be appreciated that thin,
elongated mirrors for use in photolithographic applications and
equipment may be characterized in situ through the use of
interferometer subsystems associated with the mirrors with relative
motion introduced by means of controlled motion of a translation
stage operating in a mirror characterization mode. The relative
motion may be the result of the mounting of the interferometer
subsystems on the translation stage and certain of the thin,
elongated mirrors mounted off the stage, fixedly mounted to a
reference frame, or vice versa. Once the mirrors have been
characterized, error correction signals may be used when the
apparatus is operated in a measurement mode to precisely position a
wafer with respect to the reference frame and in turn with respect
to the mask used to expose the wafer.
[0072] The feed of a laser beam to the on-stage dynamic
interferometers of FIG. 6 may be as described via source/detector
630 or by optical fibers as described by Zanoni, op. cit., or by
free space transport as described for example by Kreuzer, op. cit.,
or some combination thereof.
[0073] Based on the teachings and embodiments described
hereinabove, other variatins of the invention will be apparent to
those skilled in the relevant art and such variations are intended
to be within the scope of the claimed invention.
* * * * *